In the field of surveillance a number of sensing systems have been under study. These include chemical, sonic, seismic, radar and infrared technique. Infrared systems have proved to be particularly effective with targets that have high temperatures relative to their backgrounds. This applies to most targets of concern to the military and to large number of civilian applications as well. The best discrimination is obtained in the spectral region above 3 microns wavelength known as far infrared.
A number of approaches have been explored with regard to surveillance devices using far infrared. The most promising real time device currently available is a detector diode. These are still very expensive, but can be used to provide two dimensional images over a limited number of devices. Typical of such devices is the AN/PAS hand held thermal viewer currently used by the military. The diodes in this viewer are arranged in a row and the image reflected off of a rotary vibrating mirror with an electrical drive circuit that also supplies a horizontal synchronizing signal. A Cathode Ray Tube (CRT) type of display is hard-wired into the unit so that the operator must remain relatively close to the viewer. When a number of viewers are in use at widely separated locations the information from each is relayed by voice radio and is subject to interpretation by the operator. In a combat situation, should the operator be forced to leave the site suddenly, the viewer might have to be destroyed, even though it were carefully concealed in an inaccessible location.
At such a time the unit could easily be providing the most significant type of surveillance information. The viewer itself being a passive device is difficult to locate, but the use of a long monitoring cable would increase the chances of its discovery. The use of an FM transmitter also involves a risk, but beam forming and frequency shift techniques are available for minimizing this risk and the unit by its function permits a further decrease in the risk.